1. Field of the Invention
The present invention relates to a device for determining an abnormal degree of deterioration of a catalyst.
2. Description of the Related Art
A catalyst system is usually arranged in an exhaust passage to purify an exhaust gas. Once the catalyst system deteriorates excessively, the catalyst system cannot purify the exhaust gas sufficiently. Accordingly, it is necessary to determine an abnormal degree of deterioration of the catalyst, to inform the driver about the deterioration, and to urge the driver to exchange the catalyst for a new one.
The exchange requires much cost and much time that must be minimized. Accordingly, it is necessary to determine the abnormal degree of deterioration of a catalyst precisely.
Japanese Unexamined Patent Publication No. 7-26944 discloses a device for determining the abnormal degree of deterioration of a catalyst. The device grasps a variation of the degree of deterioration of the catalyst against the temperature thereof by using air-fuel ratio sensors. The device graphs the variation and compares the graph with a prepared threshold graph. The device determines that the degree of deterioration of the catalyst is abnormal when the graph of variation is positioned generally above the threshold graph.
The deterioration of the catalyst progresses from the upstream portion thereof to the downstream portion thereof because the upstream portion is always exposed to the non-purified exhaust gas. Accordingly, only the degree of deterioration of the upstream portion of the catalyst may be abnormal. In this case, if the temperature of the catalyst is very high and the catalyst is generally activated, the general purification ability thereof drops little. However, in the initial stage of the activation of the catalyst in which the upstream portion thereof ought to activate, the upstream portion does not activate so that the purification reaction does not start. Therefore, the catalyst is not heated by the reaction heat and the time till the catalyst is activated completely becomes long so that during this time, the purification of the exhaust gas deteriorates considerably. The above-mentioned device merely compares the variation of the degree of deterioration of the catalyst with the threshold graph. Accordingly, in such case, the device cannot accurately determine that the degree of deterioration of the catalyst is abnormal.